The present invention is in the field of automatic inspection techniques, and relates to an optical inspection method and system, particularly useful for inspecting semiconductor wafers progressing on a production line.
The manufacture of semiconductor devices consists of a multi-staged process requiring wafers progressing on a production line to be inspected between sequential manufacturing steps. One of the principle processes in the manufacture of semiconductor devices is the photolithography process. It consists of patterning the wafer""s surface in accordance with the active elements of semiconductor devices to be manufactured. The photolithography process includes the following main operational steps:
a. Coating the wafer with a photoresist material;
b. Exposing the photoresist material through a mask with a predetermined pattern in order to produce a latent image of the mask on the photoresist material; and
c. Developing the exposed photoresist material in order to produce the image of the mask on the wafer.
Generally speaking, prior to the photolithography process, the wafer is prepared for the deposition of one or more layers. After the completion of the photolithography process, the uppermost layer of the wafer is etched. Then a new layer is deposited, in order to start the photolithography and etching operations again. In this repetitive manner, a multi-layer semiconductor wafer is produced.
FIG. 1 illustrates a block diagram of a typical photocluster 1 used for performing the photolithography process in semiconductor fabrication plants (FABs). The photocluster 1 (or link) is composed of two main parts: a phototrack 2 and an exposure tool 4. The phototrack 2 includes a coater track 6 associated with a cassette load station 6a, and a developer back 8 associated with a cassette unload station 8a. Alternatively, both coating and developing functions may be combined and realized in a common station. A load/unload robot R is mounted for movement within the photocluster 1 for conveying wafers W to and from the photocluster tools. The coater track 6, the exposure tool 4 and the development track 8 are tightly joined together in order to minimize process variability and any potential risk of contamination during photolithography, which is a super sensitive process.
It is apparent that in such a complex and delicate production process various problems, failures or defects may arise or develop during each manufacturing step or from the serial combination of steps. Due to the stringent quality requirements, any defect which is not timely detected, may result in the rejection of a single wafer or the entire lot. The wafer cannot be taken out of tie photocluster tools set-up for measurement or inspection before the entire photolithography process is completed and the wafer arrives at the cassette station 8a. The wafers are typically inspected at a stand-alone monitoring system (CD-SEAM) which is installed outside the production line, and to which the wafers are transferred by means of a separate handling system. This reduces the throughput of the production line.
A manual inspection technique is conventionally used for inspecting the wafers for so-called macro lithography defects, such as scratches or foreign particles of dust and dirt. Generally, macro defects are considered as defects having tenths of micrometers in size. The manual inspection technique utilizes the visual examination of the wafers surface by an operator using intense light and magnification. This inspection is inconsistent and unreliable, since the results vary significantly amongst operators, due to the wafer""s complexity and depending on the operators"" experience. About 80% of the photo-related defects remain undetected. Manual inspection has low throughput, and could not be performed within the FAB tools. Manual inspection is not cost effective.
Various automatic inspection systems have been developed. They utilize either a line CCD camera or an area CCD camera The basic problem with line type detectors is their non-effective use of illuminating radiation. This is owing to the fact that a strip illuminated on the surface of the wafer under inspection is substantially wider than the width of the field of view of a line CCD camera. Additionally, the line CCD based technique suffers from a complicated mechanical arrangement needed for moving the line CCD camera along the X- and Y-axes relative to the wafer under inspection. Moreover, different resolutions are achieved in the X- and Y-directions, due to the movement of the image during scanning.
As for the area sensor based technique, it utilizes a so-called step-and-repeat mode of operation, wherein the camera and the wafer are mounted for movement relative to each other to cover the entire surface of the wafer. More specifically, the camera and the wafer are moved step-by-step, and images are acquired upon the camera or the wafer stops. The technique requires a quite complicated mechanical stage providing movement along two mutually perpendicular axes. Additionally, it results in low throughput due to a great number of movement steps, and requires an additional footprint for performing such a two-dimensional movement.
Machine vision systems having multiple cameras have been developed, being disclosed, for example, in U.S. Pat. No. 5,768,443. In this system, images are acquired simultaneously by a plurality of cameras. The fields of view of the cameras cover a relatively large area of the wafer under inspection, thereby increasing the throughput of the system, as compared to the techniques utilizing a single line or area sensor. However, this patent does not present any example of the cameras"" arrangement, which would be useful in an integrated inspection. An inspection machine should be inexpensive, simple to erect and maintain, and should have a small footprint (like a wafer cassette) to meet the requirements of the integrated inspection.
Laser inspection systems have been developed which utilize various types of scanners and collectors for scanning the wafer""s surface during the translational movement, and for collecting light reflective and scattered from the surface. Such systems are disclosed, for example, in U.S. Pat. Nos. 4,630,276; 5,108,176; 5,127,726 and 5,712,701. Unfortunately, these systems are complicated and expensive. Since they use monochromatic laser illumination, they are non-effective and insufficient for the inspection of developed photoresist for macro defects.
There is accordingly a need to facilitate the automatic inspection of workpieces progressing on a production line, by providing a novel system and method for the optical inspection of such workpieces.
It is a major feature of the invention that the optical inspection system may be integrated into the production line. It is simple, inexpensive and compact, and provides for inspection with a high throughput of workpieces.
The main idea of the present invention is based on the following. A semiconductor wafer is typically a substantially flat workpiece, having an axis of symmetry (i.e. is round). Consequentially, one half of the wafer can be inspected by appropriately moving respective parts of a scanning apparatus above it, and, upon rotating the wafer about its axis of symmetry by 180xc2x0, inspecting the other half of the wafer. Hence, suitable mechanics should be provided for moving the respective components of the scanning apparatus solely within an inspection area equal to the half of the wafer, rather than moving them above the entire wafer, or moving both the respective components and the wafer relative to each other. This significantly simplifies the mechanical equipment of the entire system, as well as its footprint, and enables the use of a robot typically installed in the production line for conveying wafers to and from a plurality of stations. To scan the half of the wafer (inspection area), a plurality of optical assemblies is mounted for movement above the wafer within the inspection area, the optical assemblies being aligned in a line parallel to the axis of symmetry of the wafer between the opposite sides thereof. A plurality of sensors is provided, each sensor being associated with a corresponding one of the plurality of optical assemblies. Thus, an illuminated strip extending along the entire width of the wafer is inspected at each location of the plurality of optical assemblies relative to the wafer.
There is thus provided, according to one aspect of the present invention, an optical inspection system for detecting defects on a substantially flat workpiece having an axis of symmetry, the system comprising a stage supporting said workpiece in an inspection plane, and a scanning apparatus accommodated above said workpiece, wherein:
(a) said stage is mounted for rotation in a plane parallel to said inspection plane;
(b) said scanning apparatus comprises:
an illumination assembly producing a plurality of incident radiation components, each component illuminating a corresponding one of a plurality of regions of the workpiece within a strip illuminated by said plurality of incident radiation components, the strip extending parallel to the axis of symmetry of the workpiece between two opposite sides thereof;
a plurality of optical assemblies accommodated above said workpiece, each optical assembly directing a corresponding one of a plurality of radiation components returned from the corresponding illuminated region away from the workpiece, wherein the optical assemblies are aligned along said axis of symmetry in a spaced-apart parallel relationship and mounted for reciprocating movement within a plane parallel to an inspection area, said inspection area covering substantially a half of the work-piece;
(c) a plurality of area sensors are arranged im a predetermined manner, each area sensor being associated with a corresponding one of the optical assemblies for receiving the component of the retained radiation and generating data representative thereof;
(d) a first drive mechanism is coupled to said optical assemblies for driving said reciprocating movement thereof; and
(e) a second drive mechanism is coupled to said stage for driving said rotation thereof.
The illumination assembly may operate in bright-field illumination mode. In this case, it comprises a plurality of stationary mounted light sources aligned corresponding to the alignment of the optical assemblies, each incident radiation component being directed on the corresponding optical assembly, which, in turn, directs it onto the corresponding region of the workpiece.
Alternatively, or additionally, the illumination assembly may operate in a dark-filed illumination mode. In this case, it includes either a plurality of light sources, each accommodated in the vicinity of the corresponding optical assembly, or an elongated light source, extending along the optical assemblies in the proximity thereof. This xe2x80x9cdark-fieldxe2x80x9d illumination assembly is mounted for movement together with the optical assemblies If the dark-field light illumination is used in addition to the bright-field illumination, the operation of the stationary mounted light sources and the operation of the movable light sources are time separated. For example, the stationary mounted light sources may be continuously operated, while the movable light sources are selectively actuated to inspect only those locations on the workpiece where either a potential defect has been detected during the bright-field mode, or a real defect is expected.
The area sensors may be arranged in a line parallel to the axis of symmetry and associated with the optical assemblies, so as to move together with the optical assemblies. Alternatively, the area sensors may be arranged in two parallel lies, parallel to the axis of symmetry, wherein each two adjacent sensors are displaced from each other along two mutually perpendicular axes. In this case, an additional light directing assembly is provided for receiving the components of the returned radiation ensuing from the optical assemblies, and directing them onto to the area sensors.
According to another aspect of the present invention, there is provided a photolithography tool for processing a stream of substantial flat workpieces progressing on a production line, which tool comprises an optical inspection system constructed as described above.
According to yet another aspect of the present invention, there is provided a method for inspecting a substantial flat workpiece having an axis of symmetry, the method comprising the steps of:
(i) locating the workpiece within an inspection plane;
(j) illuminating a plurality of regions of the workpiece by a plurality of incident radiation components and producing a plurality of light components returned from the plurality of illuminated regions, the illuminated regions forming an illuminated strip extending parallel to the axis of symmetry between two opposite sides of the workpiece;
(k) directing each of the plurality of light components returned from the illuminated regions through a plurality of optical assemblies aligned in a line parallel to the axis of symmetry above the workpiece;
(l) detecting light components returned from the illuminated regions by a corresponding plurality of area sensors;
(m) moving said plurality of optical assemblies relative to said workpiece within a plane parallel to the inspection planes such as to illuminate successive strips on the workpiece and detect light components returned therefrom;
(n) repeating steps (j) to (n) for skip-by-strip inspection of the workpiece within an inspection area that covers substantially a half of the workpiece;
(o) rotating said workpiece by 180xc2x0 and repeating steps (j) to (o).
More specifically, the present invention is used for inspecting wafers progressing within a photolithography tools set-up, and is therefore described below with respect to this application.